Archive for Milky Way

The Planck Sky

Posted in The Universe and Stuff with tags , , , , , , , on July 5, 2010 by telescoper

Hot from the press today is a release of all-sky images from the European Space Agency’s Planck mission, including about a year’s worth of data. You can find a full set of high-resolution images here at the ESA website, along with a lot of explanatory text, and also here and here. Here’s a low-resolution image showing the galactic dust (blue) and radio (pink) emission concentrated in the plane of the Milky Way but extending above and below it. Only well away from the Galactic plane do you start to see an inkling of the pattern of fluctuations in the Cosmic Microwave Background that the survey is primarily intended to study.

It will take a lot of sustained effort and clever analysis to clean out the foreground contamination from the maps, so the cosmological interpretation will have to wait a while. In fact, the colour scale seems to have been chosen in such a way as to deter people from even trying to analyse the CMB component of the data contained in these images. I’m not sure that will work, however, and it’s probably just a matter of days before some ninny posts a half-baked paper on the arXiv claiming that the standard cosmological model is all wrong and that the Universe is actually the shape of a vuvuzela. (This would require only a small modification of an earlier suggestion.)

These images are of course primarily for PR purposes, but there’s nothing wrong with that. Apart from being beautiful in its own right, they demonstrate that Planck is actually working and that results it will eventually produce should be well worth waiting for!

Oh, nearly forgot to mention that the excellent Jonathan Amos has written a nice piece about this on the BBC Website too.

Planck and the Cold Galaxy

Posted in The Universe and Stuff with tags , , , , , , on March 17, 2010 by telescoper

Just a quick post to show a cool result from Planck which has just been released by the European Space Agency (ESA). It will be a while before any real cosmological results are available, but in the meantime here are a couple of glimpses into the stuff we cosmologists think of as foreground contamination but which are of course of great interest in themselves to other kinds of astronomers.

The beautiful image above (courtesy of ESA and the HFI Consortium) covers a portion of the sky about 55 degrees across. It is a three-colour combination constructed from Planck’s two shortest wavelength channels (540 and 350 micrometres, corresponding to frequencies of 545 and 857 GHz respectively), and an image at 100 micrometres obtained with the Infrared Astronomical Satellite (IRAS). This combination effectively traces the dust temperature: reddish tones correspond to temperatures as cold as 12 degrees above absolute zero, and whitish tones to significantly warmer ones (a few tens of degrees above absolute zero) in regions where massive stars are currently forming. Overall, the image shows local dust structures within 500 light years of the Sun.

Our top man in the HFI Consortium,  Professor Peter Ade, is quoted as saying

..the HFI is living up to our most optimistic pre-flight expectations.  The wealth of the data is seen in these beautiful multicolour images exposing previously unseen detail in the cold dust components of our galaxy.  There is much to be learned from detailed interpretation of the data which will significantly enhance our understanding of the star formation processes and galactic morphology.

This Planck image was obtained during the first Planck all-sky survey which began in mid-August 2009. By mid-March 2010 more than 98% of the sky has been observed by Planck. Because of the way Planck scans the sky 100% sky coverage for the first survey will take until late-May 2010.

Other new results and a more detailed discussion of this one can be found here and here.

The Chromoscope

Posted in The Universe and Stuff with tags , , , , on December 5, 2009 by telescoper

Just a quick post to plug the chromoscope, which is “an accessible, easy tool that anyone can use to explore and understand the sky at multiple wavelengths”. It was originally created for the Royal Society Summer Science Exhibition 2009 by Stuart Lowe (Jodrell Bank), Chris North (Cardiff), and Robert Simpson (Cardiff) and is now available online for your education and enjoyment.

It has its own blog on which there’s a load of information about  all the different data sets used to make it (covering the range from radio to X-ray), and there’s even a video to explain how it works so I don’t have to!

I was there for part of the Summer Exhibition (I blogged about it, in fact) so had the chance to play with the original version, which was set up for  large display screens on the Herschel/Planck exhibit. Have a go with it yourself on the small screeen by clicking here!

The Milky Way in a New Light

Posted in The Universe and Stuff with tags , , , , , on October 2, 2009 by telescoper

I note that the Herschel mission now has its own blog, so I no longer have to try to remember to put all the sexy images on here. However, at the end of a worrying week for UK astronomy, I thought it would be a good idea to put up one of the wonderful new infra-red images of the Milky Way just obtained from Herschel. This is the first composite colour picture made in “parallel mode”, i.e. by using the PACS and SPIRE instruments together. Together the two instruments cover a wavelength range from 70 to 500 microns. The resulting image uses red to represent the cooler long-wavelength emission (seen by SPIRE) and bluer colours show hotter areas. The region of active star formation shown is close to the Galactic plane; detailed images such as this, showing the intricate filamentary structure of the material in this stellar nursery, will help us to understand better how what the complex processes involved in stellar birth.

In a Galaxy, Faraday…

Posted in The Universe and Stuff with tags , , , , on July 21, 2009 by telescoper

I was finishing off the draft of a paper the other day and remembered a little paper I did some time ago with a former PhD student of mine, Patrick Dineen. I thought it would be fun to put the pictures up here because it was one of those occasions when a little idea turns out much nicer than you expected…

What we had to start with was a collection of Faraday Rotation measurements of extragalactic radio sources dotted around the sky. Their distribution is fairly uniform but I hasten to add that it was not a controlled sample so it would be not possible to take the sources as representative of anything for statistical purposes.

Faraday rotation occurs because left and right-handed polarizations of electromagnetic radiation travel at different speeds along a magnetic field line. The effect of this is for the polarization vector to be rotated as light waves travel and the net rotation angle (which can be either positive or negative) is related to the line integral of the component of the magnetic field along the line of sight travelled by the waves. The picture below shows the distribution of sources, plotted in Galactic coordinates and coded black for negative and white for positive.

rotation

Some radio galaxies have enormously large Faraday rotation measures because light reaches us through regions of the source that have strong magnetic fields. However, for most sources in our sample the rotation measures are smaller and are thought to be determined largely by the propagation of light not through the emitting galaxy, near the start of its journey towards us, but through our own Galaxy, the Milky Way, which is near the end of its path.

If this is true then the distribution of rotation measures across the sky should contain information about the magnetic field distribution inside our own Galaxy. Looking at the above picture doesn’t give much of a hint of what this structure might be, however.

What Patrick and I decided to do was to try to make a map of the rotation measure distribution across the sky based only on the information given at the positions where we had radio sources. This is like looking at the sky through a mask full of little holes at the source positions. Using a nifty (but actually rather simple) trick of decomposing into spherical harmonics and transforming to a new set of functions that are orthogonal on the masked sky we obtained the following map:

uni_16_rmjoint

(The technical details are in the paper, if you’re interested.) You probably think it looks a bit ropey but, as far as I’m concerned, this turned out stunningly well. The most obvious features are a big blue blob to the left and a big red blob to the right, both in the Galactic plane. What you’re seeing in those regions is almost certainly the local spur (sometimes called the Orion Spur; see below), which is a small piece of spiral arm in which the local Galactic magnetic field is confined. The blobs show the field coming towards the observer on one side and receding on the other. The structure seen is relatively local, i.e. within a kiloparsec or so of the observer.

I was very pleased to see this come out so clearly from an apparently unpromising data set, although we had to confine ourselves to large-scale features because of instabilities in the reconstruction of high-frequency components.

What’s all the Noise?

Posted in Science Politics, The Universe and Stuff with tags , , , , on January 18, 2009 by telescoper

Now there’s a funny thing…

I’ve just come across a news item from last week which I followed up by looking at the official NASA press release. I’m very slow to pick up on things these days, but I thought I’d mention it anyway.

The experiment concerned is called ARCADE 2, which is an somewhat contrived acronym derived from Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission. It is essentially a balloon-borne detector designed to analyse radio waves with frequencies in the range 3 to 90 Ghz. The experiment actually flew in 2006, so it has clearly taken considerable time to analyse the resulting data.

Being on a balloon that flies for a relatively short time (2.5 hours in this case) means that only a part of the sky was mapped, amounting to about 7% of the whole celestial sphere but that is enough to map a sizeable piece of the Galaxy as well as a fairly representative chunk of deep space.

There are four science papers on the arXiv about this mission: one describes the instrument itself; another discusses radio emission from our own galaxy, the Milky Way; the third discusses the overall contribution of extragalactic origin in the frequency range covered by the instrument; the last discusses the implications about extragalactic sources of radio emission.

The thing that jumps out from this collection of very interesting science papers is that there is an unexplained, roughly isotropic, background of radio noise, consistent with a power-law spectrum. Of course to isolate this component requires removing known radio emission from our Galaxy and from identified extragalactic sources, as well as understanding the systematics of the radiometer during its flight. But after a careful analysis of these the authors present strong evidence of excess emission over and above known sources. The spectrum of this radio buzz falls quite steeply with frequency so appears in the two long-wavelength channels at 3 and 8 GHz.

So where does this come from? Well, we just don’t know.

The problem is that no sensible extrapolation of known radio sources to high redshift appears to be able to generate an integrated flux equivalent to that observed. Here is a bit of the discussion from the paper:

It is possible to imagine that an unknown population of discrete sources exist below the flux limit of existing surveys. We argue earlier that these cannot be a simple extension of the source counts of star-forming galaxies. As a toy model, we consider a population of sources distributed with a delta function in flux a factor of 10 fainter than the 8.4 GHz survey limit of Fomalont et al. (2002). At a flux of 0.75 μJy, it would take over 1100 such sources per square arcmin to produce the unexplained emission we see at 3.20 GHz, assuming a frequency index of −2.56. This source density is more than two orders of magnitude higher than expected from extrapolation to the same flux limit of the known source population. It is, however, only modestly greater than the surface density of objects revealed in the faintest optical surveys, e.g., the Hubble Ultra Deep Field (Beckwith et al. 2006).  The unexplained emission might result from an early population of non thermal emission from low-luminosity AGN; such a source would evade the constraint implied by the far-IR measurements.

The point is that ordinary galaxies produce a broad spectrum of radiation and it is difficult to boost the flux at one frequency without violating limits imposed at others. It might be able to invoke Active Galactic Nuclei (AGN) to do the trick, but I’m not sure. I am sure there’ll be a lot work going on trying to see how this might fit in with all the other things we know about galaxy formation and evolution but for the time being it’s a mystery.

I’m equally sure that these results will spawn a plethora of more esoteric theoretical explanations, inevitably including the ridiculous as well as perhaps the sublime. Charged dark matter springs to mind.

Or maybe it’s not even extragalactic. Could it be from an unknown source inside the Milky Way? If so, it might shed some light on the curiosities we find in the cosmic microwave background that I’ve mentioned here and there, but it seems to peak at too low a frequency to account for much of the overall microwave sky temperature.

But it does have a lesson for astronomy funders. ARCADE 2 is a very cheap experiment (by NASA standards). Moreover, the science goals of the experiment did not include “discovering a new cosmic background”. It just goes to show that even in these times of big, expensive and narrowly targetted missions there is still space for serendipity.